1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which fixes a toner image by a fixing unit.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming apparatus such as a duplicating machine, a printer or a facsimile machine, when an image is formed on a sheet, a toner image formed in an image forming unit is transferred onto the sheet. Then, the toner image is heated, pressurized and fixed onto the sheet by a fixing unit, so as to form the image onto the sheet.
As the image forming apparatus, there is an image forming apparatus including a re-transport unit which reverses the sheet with the toner image fixed to a first side of the sheet, and transports the sheet to the image forming unit again. In duplex printing in which images are formed on both sides of the sheet, the sheet with the image formed on the first side of the sheet is reversed and transported to the image forming unit again by the re-transport unit. As a result, the image is formed on a reverse side of the sheet.
As the fixing unit, there is a fixing unit using a thermal-pressure fixing system. This fixing unit includes a fixing roller and a pressure roller, which simultaneously apply heat and pressure to the sheet so as to fix the toner image to the sheet. In a case of this fixing unit, when the toner image is fixing to the sheet, the fixing roller, which heats the sheet, gives a substantial amount of heat to the sheet. Thus, when the toner image is fixed, moisture contained in the sheet evaporates into water vapor.
After the water vapor is generated, when a main body of the image forming apparatus is set in a relatively low temperature state, the water vapor may condense within a sheet-transport path. If dew condensation occurs, when the sheet passes through the sheet-transport path, water droplets adhere to the sheet. In the conventional image forming apparatus, in order to prevent this dew condensation, the air-tightness of the fixing unit is increased so as to absorb the water vapor within the fixing unit. In addition, for example, there is an image forming apparatus which discharges the generated water vapor out of the apparatus through a louver provided on an upper portion of the image forming apparatus (see Japanese Patent Application Laid-Open No. H08-254938).
As operations of the conventional image forming apparatus have been accelerated in recent years, the amount of heat transferred from the fixing roller to the sheet has also increased, and thus the amount of the generated water vapor itself has also increased. Also, in a conventional configuration in which the air-tightness of the fixing unit is increased so as to absorb the water vapor within the fixing unit, the absorption of the water vapor is limited, and collection of the water vapor has become difficult.
If the water vapor is not sufficiently discharged and collected, when the main body of the image forming apparatus is set in the relatively low temperature state, the water vapor condenses on a guide member which guides the sheet to be reversed, in the duplex printing, for example. In recent years, in order to downsize the image forming apparatus and to improve productivity in the duplex printing, a pair of switchback rollers, provided in the re-transport unit so as to reverse and transport the sheet, may be arranged near the fixing unit. In this case, the water vapor also condenses on surfaces of the pair of switchback rollers.
When this dew condensation occurs, for example, as in a cold start, if duplex printing is attempted in a state where the main body of the image forming apparatus is not sufficiently heated up, the water vapor condensing on the guide member and the pair of switchback rollers adheres as the water droplets to the sheet to be reversed and transported. When the water droplets adhere, the electrical resistance value at a portion of the surface of the sheet at which the water droplets adhere decreases relative to a surrounding area without the adherence of the water droplets on the surface of the sheet. Thus, when the toner image is transferred by a transfer unit, a poor image that includes, for example, blurring or the like, due to poor transfer may be generated, or wrinkles or roughness of the sheet may be caused.
For example, if first-side printing is continuously performed in a cold state, the moisture contained in the sheet becomes water vapor due to the heat from the fixing unit, and the water vapor adheres to a pair of reverse rollers located above the fixing unit. If the duplex printing is set after first-side printing, the moisture, which has adhered to the pair of switchback rollers in the first-side printing, adheres to the sheet to be transported for performing the duplex printing.